1. Field of the Invention
The present invention relates in general to ventilation systems in breathing apparatuses, such as anesthesia apparatuses, and in particular to a ventilation system with a mechanical ventilation system combined with a manual ventilation system. The invention provides a breathing apparatus and also a method for controlling a breathing apparatus, especially an electronic expiration valve in the apparatus. The invention also concerns software products for controlling a breathing apparatus.
2. Background and Prior art
When patients are subjected to anesthesia there is usually a transition from ventilation by spontaneous breath, via a phase of manually controlled ventilation by means of a manual breathing bag over to mechanically controlled ventilation, and vice versa when the patient is taken out of anesthesia. A direct transition from spontaneous to mechanical ventilation is considered to be too harsh for the patient and it is important to closely monitor the patient's response to the anesthesia. A human operator executing manually controlled ventilation with a direct contact between the breathing bag and the lungs of the patient is more sensitive to the conditions and the reactions of the patient than the mechanical ventilation system and can adjust anesthesia parameters in a smother and safer way. Also during mechanical ventilation operators sometimes want to switch over to a phase of manually controlled ventilation in order to check the condition of the patient, for example in connection with a change in the composition of an anesthetic gas or in connection with distribution of an anesthetic agent.
Anesthesia apparatuses are therefore usually provided with a manual ventilation system in parallel with an automatic mechanical ventilation system, and a ventilation selection switch for selecting between the manual and the mechanical ventilation system. Although having tubing, valves and other components in common in a breathing circuit connected to the lungs of a patient, the manual and the mechanical ventilation systems are basically separate systems with separate pressure control valves designed for different purposes and functions.
In the manual ventilation system an important control valve is the adjustable pressure limit valve, commonly called APL valve. The APL valve has the function to limit the pressure of breathing gas that can occur in the breathing circuit during manual ventilation. Traditionally, the APL valve is provided with a spring that exerts pressure on a diaphragm that seals off a vent passage against a valve seat. When the pressure exceeds the spring force, the APL valve opens to vent excess gas into an evacuation system. The valve is adjusted by compressing the spring with a screw mechanism so that the level of the compressed spring force corresponds to the wanted pressure limit.
FIG. 2A-FIG. 2C show schematically pressure and flow characteristics in a breathing circuit, with a prior art APL valve drawn as graphs of pressure and flow parameters over time in an exemplifying case of operation. FIG. 2A shows the system pressure in the breathing circuit Psys over time t, with the indicated level APL that is preset on the adjustable pressure limit valve. FIG. 2B shows the compression rate of the manual bag over time t, which for example would correspond to or can be described as the change rate in the volume of the manual bag (time derivative of bag volume). FIG. 2C shows the flow of gas Qout over time that is let out from the system in this instance via the APL valve. FIG. 2C also shows the flow Qpat over time to and from the patient. In FIG. 2C, the flow level Qf is the flow level of the fresh gas flow, which usually is a selectable and adjustable constant flow. Thus, in the time interval from 0 to T1 the patient inspires a part Qpat of the fresh gas flow Qf and the rest of the fresh gas flow builds up the pressure Psys until the APL pressure level is attained at T1. At T1 the APL valve opens and lets out a gas flow corresponding to the fresh gas flow level Qf, and at the same time the flow to the patient ceases. The manual bag is now filled at the APL pressure level. At T2 the operator compresses the manual bag which results in an increase in the flow Qout from the breathing circuit since the APL pressure level is already attained. No gas flow to the patient is induced by the bag compression from T2 to T4, which rather has the purpose of adjusting the volume in the manual bag by pressing out superfluous fresh gas from it. At T4 the compression of the manual bag is also released and the patient starts an expiration phase that lasts until T5. At T5 an inspiration phase begins. At T6 the operator starts manual bag compression and induces an increased gas flow Qpat to the patient until T7. At T7 the APL pressure is attained whereupon the flow Qpat to the patient ceases and the outlet pressure Qout starts and lasts until T8. At T8 the manual bag compression is released and the patient starts an expiration phase that lasts until T9 during which the outlet flow Qout ceases and the manual bag is filled with the gas expired from the patient. In FIG. 2C the changes in the flow curves Qpat and Qout coincide at T1 and T7, respectively, but for visibility reasons the flow curves are drawn with a gap in between.
Another valve type commonly used in manual ventilation is the Berner valve described in U.S. Pat. No. 3,780,760. FIG. 3A-3C show in a similar way the characteristics of such a prior art Berner valve. FIG. 3A shows the system pressure in the breathing circuit Psys over time t, with the indicated level PBern that is preset on the Berner valve for a pressure level to be maintained as long as there is no compression of the manual bag. FIG. 3B shows the compression rate of the manual bag over time t. FIG. 3C shows the flow of gas Qout over time that is let out from the system in this instance via the Berner valve. FIG. 3C also shows the flow Qpat over time to and from the patient. In FIG. 2C, the flow level of the fresh gas flow is indicated as Qf. Thus, in the time interval from 0 the patient inspires a part Qpat of the fresh gas flow Qf until the Psys attains the PBern pressure level, whereupon the Berner valve opens and lets out a flow Qpat. The flow to the patient ceases until T1 where compression of the manual bag starts and the Berner valve is mechanically triggered to allow a pressure that exceeds the PBern pressure level. From T1 to T2 a relatively high flow Qpat flows to the patient and from T2 it decreases to and remains at the lower fresh gas flow level Qf as the operator holds the manual bag at a constant volume until T3. At T3 the operator releases the bag compression and an expiration phase is started, which results in a flow Qpat from the patient to the manual bag followed by an increase up to Qf level in the outlet flow Qout from the breathing circuit. In FIG. 3C the indicated area between the curves Qout and Qpat corresponds to the gas volume in the bag. At T5 the pressure level PBern is attained and maintained until T6. At T6 the manual bag is compressed, this time to less degree than the previous compression, and again the pressure Psys increases and there is again a two step first high then lower flow Qpat to the patient. At T8 an expiration phase starts and proceeds with the same pattern as before. With such a Berner valve, there is a risk that the pressure increases to a too high a level with an entailing risk for injuries on the patient, such as barotraumas. Further, if the fresh gas flow Qf exceeds a mechanical trigger level, there is that the fresh gas flow Qf is mistakenly interpreted as a breath.
When the mechanical ventilation mode is set, the APL valve or the Berner valve is no longer a part of the breathing circuit. The mechanical ventilation system operates, in the absence of the APL valve or corresponding valve, with an expiration valve for venting excess gas into the evacuation system. In prior art breathing apparatuses the expiration valve is electronically controlled not only to limit the maximum pressure that should occur in the breathing circuit but also to ensure a minimum pressure in the breathing circuit. This minimum pressure is commonly known as the positive end expiratory pressure PEEP and is important to ensure that the lungs of a patient always to some extent are filled with breathing gas in order not to collapse and be completely deflated. The expiration valve is therefore usually called a PEEP valve. The PEEP valve is flexibly operated via the normal user interface of the breathing apparatus and is usually controlled by means of a control computer program realizing a set of predetermined rules adapted to the requirements of the mechanical ventilation mode.
The manual ventilation system of such breathing apparatuses is less flexible and does not allow for very accurate control of the pressure in the breathing circuit. There is therefore a need for improvements in the manual ventilation system of breathing apparatuses having both a manual and an automatic mechanical ventilation system.
There are different examples of prior art showing breathing apparatuses with manual and mechanical ventilation systems.
WO 2004/067055 A2 shows an example of an open ventilation system of the type described above with a manual ventilation system, an automatic mechanical ventilation system and a ventilation selection switch to select between the manual and the mechanical systems. This piece of prior art is directed to such a ventilation system in which the number of components that must be autoclaved are reduced. The use of a CO2 absorber, an APL valve or Berner valve is eliminated. A selection valve for connecting either of the manual or the mechanical ventilation system is provided outside the patient circle. The gases from the patient are prevented from returning to the manual bag in order to eliminate the need for autoclaving the manual bag. The automatic mechanical ventilation system and the manual ventilation system have separate expiration valves for the outlet of exhalation gas to atmosphere.
In the manual ventilation mode fresh inhalation gas is input to the manual bag via a bag filling valve that is devised with an adjustable bias spring in order to allow a gas flow to the bag dependent on a differential pressure between the inhalation gas source and the manual bag. This bag filling valve has the function to limit the pressure in the manual bag. When the manual bag is manually compressed breathing gas flows via an inhalation conduit through a patient input branch of a Y-piece connected to the airways of the patient. In the exhalation phase, exhalation gas from the patient flows through a patient output branch of the Y-piece via an expiration valve selector that in the manual ventilation mode is open for evacuation of exhalation gas through a manual expiration valve to atmosphere. The maximum pressure of the manual bag is controlled independently of the pressure in the airways of the patient and exhalation gas is simply let out through the manual ventilation expiration valve, and therefore there is no need for an APL valve or a Berner valve in this piece of prior art.
In the mechanical ventilation mode, fresh inhalation gas flows past the manual bag branch, which is closed by means of the ventilation selection switch, to the patient via the Y-piece in the same manner. In the exhalation phase, exhalation gas flows via the expiration valve selector that in the mechanical ventilation mode is open for evacuation of exhalation gas through an automatic ventilation expiration valve. A separately controlled automatic ventilation mode PEEP valve is provided in the mechanical ventilation system.
U.S. Pat. No. 5,471,979 discloses an example of a breathing circuit that is coupled in a circle system and arranged for re-use of anesthetic gases that are not absorbed by the patient. This piece of prior art shows an entirely mechanical ventilation system and there is neither any manual bag nor any APL valve.